Columb Strack, senior analyst at IHS, said: "There are early indications that the group is struggling to balance its budget, with reports of cuts to fighters’ salaries, price hikes on electricity and other basic services, and the introduction of new agricultural taxes.

"While the Islamic State’s refining capacity has been largely destroyed, there is likely to be some reluctance to completely destroy oil wells, given the risk of irreversible damage to the fields, and the potential environmental impact."